1. Field of the Invention
The present invention is directed to the field of alternator generator construction and more specifically to the area of reducing noise generated in such devices.
2. Description of the Prior Art
In alternators and generators of conventional design, such as those shown in U.S. Pat. Nos. 3,538,362 and 4,835,428 the metal housing shells which enclose the stator, rotor and bearings are usually held together with bolts extending in a direction parallel to the axis of the rotor. Bolts are inserted through aligned apertures formed in the two housings so that the threads of each bolt are mated with threads in one of the apertures. The head of each bolt applies pressure to a flush surface below its circular flange that forces the two housing shell pieces into compression.
Excessive noise problems have been encountered with that type of alternator housing connection technique due to back emf pulsation pressures present on the stator during certain speeds of rotation. Specifically, it has been found that at certain speeds the bolts are caused to vibrate at their own resonant frequencies. When this occurs, the bolt head is caused to go into lateral oscillatory motion as shown in FIG. 1. In that drawing, the tooled mass which forms the bolt head 144 is shown with a circular flange 146 that is tightened to be in contact with the surface of housing 116. During most operational speeds of the associated alternator, bolt head 144 remains in the position p. At a rotational speed of approximately 1100 RPM, the through bolt 140 vibrates at its peak resonance and bolt head 144 oscillates between p' and p". In FIG. 4, the 36th order harmonic noise attributed to the through bolt resonance vibration is plotted in the heavy line. The plot shows an approximately 68 DB peak at about 1100 rpm. With a frequency of approximately 660 Hz, the vibrations are well within the audio range. It is most desireable to reduce or eliminate such noise.